THE WORST summer smog conditions of the year were experienced across large parts of England yesterday, and pollution levels threaten to be even higher today, exacerbated by extra traffic on the roads because of the railway strike.

Only a break in the weather will disperse the photochemical smog, caused mainly by fumes from petrol and diesel vehicles, combined with still atmospheric conditions and uninterrupted strong sunshine. Light easterly winds added to the problem by wafting in pollution from Europe.

Thundery showers are forecast for the south and east of England this afternoon, where smog levels have been high. While thunderstorms disperse the pollution, they also cause serious asthma problems in summer because they release pollen and fungi spores.

Monday's top temperature, 31C (88F) recorded at Northolt in north-west London, was topped by a measurement of 32.6C (91F) at Heathrow airport yesterday afternoon. It was the hottest July day in five years. By tomorrow, however, the temperature should have slipped to about 25C (77F).

This is the second smog episode this month. On 2 July, air quality across much of England and Wales was deemed to be 'poor' under the Government's banding system.

For ozone, the most important summer smog pollutant, the poor band starts at a concentration in air of 90 parts per billion. Instruments at government monitoring stations in East Anglia and Oxfordshire recorded levels slightly above this level, with the highest reading of 107ppb detected in Derbyshire. Other stations in the south-east and central England picked up concentrations approaching 90 ppb.

Labour and Friends of the Earth attacked the Government for putting out only a low-key warning on Monday, suggesting drivers should 'consider carefully whether use of their cars was necessary'. With the railways almost shut down today and the Automobile Association forecasting a surge in road traffic, the plea is unlikely to have any impact.

Labour yesterday launched cleaner air policies advocating a long-term switch from private to public transport plus more immediate measures to cut air pollution. The party's environment spokesman, Chris Smith, said councils should be given powers to reduce car traffic and promote cycling, walking and public transport in order to achieve local air quality standards. While the Government would set a minimum standard, councils could choose a higher one if they wished.

He said pilot schemes of 'pollution charging' where cars would be charged for entering city centres should be tried, and pollution cameras could be used to identify the relatively small number of badly tuned and maintained vehicles which contribute disproportionately to pollution. Measures to speed up the fitting of catalytic converters were also needed.

Dr John Ayres, a Birmingham chest consultant who advises the Government and the National Asthma Campaign, estimated that about a million asthma sufferers were likely to experience extra discomfort and wheeziness because of the high levels of ozone and nitrogen dioxide. Some people with chest illnesses such as chronic bronchitis were also affected.